1. Field of the Invention
The present invention relates to a process for preparing an aqueous solution of sulfonated melamine-formaldehyde resin, and a cement composition in which the solution and a setting retarder are added to a concrete, a mortar, a cement paste or the like.
2. Description of the Related Art
An aqueous solution of sulfonated melamine-formaldehyde resin has conventionally been used as a dispersant for cement for the purpose of increasing fluidity of a cement composition such as a concrete or a mortar, or decreasing a unit amount of water thereby increasing strength and durability.
Some proposals relating to the improvement in a process for preparing such a water reducing agent have been disclosed.
JP-A-55-7590 discloses a melamine-urea formaldehyde condensate modified with an aromatic aminosulfonic acid. Further, its molar ratio is defined such that the aromatic aminosulfonic acid to the total of melamine and urea is (0.2-1.0):1.0, and the molar ratio of melamine to urea is defined 0.75:0.25 to 0.30:0.70.
JP-B-1-40850 discloses a process for preparing an aqueous solution of melamine/aldehyde resin, which comprises: step (a) of reacting 1 mole of melamine, 3.5-6 moles of formaldehyde and 0.5-1 moles of a compound 1 (wherein the compound 1 is at least one member selected from the group consisting of a sulfite of an alkali metal, a sulfite of an alkaline earth metal, a sulfate of an alkali metal, a sulfate of an alkaline earth metal, aminosulfonic acid, amino acid, aminodicarboxylic acid, hydroxycarboxylic acid, hydroxycarboxylic acid lactone, sulfanilic acid and sulfamic acid) in an alkaline medium having a pH of 9.0-13.0 in which water is present, at a temperature of 75-90.degree. C.; step (b) of reacting the mixture obtained in the step (a) with 0.1-3 moles, per mole of melamine used in the step (a), of a compound 2 (wherein the compound 2 is at least one member selected from the group consisting of amino acid, aminocarboxylic acid, aminodicarboxylic acid, carboxylic acid, hydroxycarboxylic acid, hydroxycarboxylic acid lactone, sulfamic acid, sulfanilic acid, aminosulfonic acid, polyhydroxycarboxylic acid and polyhydroxycarboxylic acid lactone at a pH of 5.5-6.5 and a temperature of 75-90.degree. C.; step (c) of adding a basic compound 3 (wherein the basic compound 3 is at least one member selected from the group consisting of an amine, a polyamine, an oxide of an alkali metal, a hydroxide of an alkaline metal, ammonium hydroxide, an oxide of an alkaline earth metal, monoethanol amine, diethanol amine, triethanol amine, a basic salt of a transition metal and a basic salt of zinc or aluminum) to the resin solution obtained in the step (b) to adjust a pH of the solution to 8.0-13.0, and then cooling the solution obtained in the step (c).
DE 441197 discloses a process for preparing an aqueous solution of sulfonated melamine-formaldehyde resin by adjusting the ratio of melamine, sulfanilic acid and formaldehyde to 1:1.1-1.5:3.3-3.6, adjusting a pH of a mixed solution to 5.0-7.0, heating the solution at 50-90.degree. C., and adjusting a viscosity to 10-60 cSt (80.degree. C.).
Conventional water reducing agents comprising a sulfonated melamine-formaldehyde resin as a main component have had the defect in that time for retaining fluidity of a cement composition is short, that is, the fluidity lowers with the passage of time. For this reason, the water reducing agent is mainly used in a concrete secondary product factory, it is used as a fluidizing agent in a ready-mixed concrete, and it is not well used in a ready-mixed concrete production plant. In particular, this tendency is strong in Japan.
In JP-A-55-7590, JP-B-1-40850 and DE 441197, it is difficult to prevent decrease of the fluidity, and it is therefore difficult to use in the ready mixed concrete production plant.